The field of the invention is digital controllers, and more particularly, programmable controllers used to operate industrial machines and processes.
Programmable controllers such as that disclosed in U.S. Pat. No. 3,810,118 issued May 7, 1974, and entitled "Programmable Matrix Controller" are rapidly replacing relay panels and hardwired logic systems for many control applications. Sensing devices such as limit switches are mounted on the controlled system and connect to the programmable controller to provide information concerning the status of the controlled system. The programmable controller also connects to operating devices, such as solenoids and motors on the controlled system, and depending on the status of the system as indicated by the sensing devices, it selectively energizes the operating devices to provide the desired system operation. The desired system operation is determined by a control program which is comprised of a set of instructions which are sequentially read out of a controller memory.
As disclosed in U.S. Pat. No. 3,942,158, entitled "Programmable Logic Controller", the speed at which the control program is executed can be substantially increased if the input/output operations are performed independently of the controller processor by an I/O scanner circuit. The I/O scanner circuit operates to periodically couple status data between the controlled system and input/output image tables stored in the controller memory. The controller processor operates at high speed to execute a control program which is also stored in the controller memory and to thereby continuously update the the output image table. The asynchronous operation of the I/O scanner circuit allows state of the art processing speeds for execution of the control program while maintaining relatively low data rates in the circuitry which interfaces the programmable controller to the controlled system.
In the vast majority of present applications the programmable controller is mounted in a cabinet situated immediately alongside the machine being controlled. A pair of wires from each sensing device and each operating device on the controlled machine are connected to interface circuitry which is located on an I/O interface rack mounted inside the cabinet. Such an I/O interface rack is described in the above cited copending patent application as well as in copending U.S. patent application Ser. No. 562,247, filed Mar. 26, 1975 and entitled "Disconnect Arm for Electrical Equipment". The number of I/O interface racks for any installation will depend upon the complexity of the machine being controlled and each I/O interface rack is connected to the I/O scanner circuit by means of a short cable. Data is coupled between the I/O interface racks and the I/O scanner circuit over a very short distance and within the confines of a cabinet.
Since their introduction in 1969 programmable controllers have been limited by economic considerations to relatively complex control applications. It has long been recognized that one means of overcoming this economic limitation is to use a single programmable controller to operate more than one machine, and to thereby more fully utilize the capabilities of the controller processor and the full capacity of the controller memory. The difficulty, however, is that the machines to be controlled are often spread throughout the factory and the programmable controller must be located remotely from them. Such an arrangement requires the transmission of numerous digital logic signals over long distances through a noisy industrial environment. Also, it is often desirable to operate controlled machines independently of one another so that if one should be shut down due to a malfunction, for example, it does not necessarily affect the remaining controlled machines.